Electrically-controlled, micro-machined "mirrors" can be used to alter the path of an optical signal. Such mirrors are usually implemented as metallized layer of polysilicon or as a dielectric stack. Among other applications, such mirrors can be used to create reconfigurable optical networks wherein one or more optical signals from one or more source fibers are directed to any one of several destination fibers via operation of the mirror. Such an arrangement, wherein an optical element (e.g., a mirror) is adjusted, typically in response to a sensed condition, is commonly referred to as "adaptive optics."
In one conventional adaptive optics arrangement, a reflective layer having a uniform thickness is suspended above an electrode. As a voltage is applied across the reflective layer and the electrode (hereinafter "actuation"), the reflective layer deforms. An optical signal incident on the reflective layer is directed to a different destination on reflection as a function of the deformed or undeformed shape of the reflective layer.
A simplified schematic of such an arrangement is depicted FIG. 1, wherein reflective layer or mirror 102 is suspended, via supports 104, over electrode 106. Both mirror 102 and electrode 106 are substantially parallel to substrate surface 108. Optical fibers 110, 112 and 114 are in optical communication with mirror 102.
In the arrangement depicted in FIG. 1, the path that an optical signal follows upon reflection from mirror 102 is dictated by the shape of the mirror. That relationship is illustrated in FIGS. 2a and 2b. In FIGS. 2a and 2b, optical fibers 110 and 112 deliver respective optical signals 116 and 118 to mirror 102. When the mirror is undeformed such that it has a flat form, as depicted in FIG. 2a, optical signals 116 and 118 delivered to mirror 102 from respective optical fibers 110 and 112 are returned to those optical fibers upon reflection. On the other hand, when mirror 102 is deformed such that it has a curved form, as depicted in FIG. 2b, optical signals 116 and 118 delivered to the mirror are reflected to optical fiber 114, rather than to the source fibers 110 and 112.
Mirror 102 is deformed by applying a voltage across the mirror and electrode 106. The applied voltage generates an electrostatic force that causes mirror 102 to move towards electrode 106. Since the ends of mirror 102 are immobilized, the mirror deforms in a characteristically parabolic shape. When the voltage is removed, the electrostatic force diminishes, and mirror 102 substantially returns to its flat, undeformed shape.
As is clear from the foregoing description of the arrangement depicted in FIG. 1, the path that an optical signal follows upon reflection from mirror 102 is dictated by the shape of the mirror. And, the shape of mirror 102 depends upon the mechanical response of the uniform-thickness reflective layer serving as the mirror. Thus, the optical and mechanical response or properties of the mirror are disadvantageously coupled (i.e., they are not independent of one another). Moreover, the mechanical response of such a uniform layer is difficult to precisely control. In view of the extremely severe tolerances required for directing optical signals among fibers, particularly single-mode fibers (ie., about 1 micron tolerance), the utility of such a device is limited.
A second conventional adaptive optics arrangement is a mirror array comprising a plurality of individually-controlled discrete mirror elements. The optical behavior of the mirror array is dictated by its surface features, which is a function of the state (e.g., orientation, shape, etc.) of the plurality of individual mirror elements comprising the array. Thus, by individually controlling the mirror elements through the action their associated actuators, the surface features of the array can be varied to obtain a desired optical response.
A variety of actuators can be used in such an arrangement. One type of actuator is depicted in FIG. 3, which shows a single mirror element 322 connected to actuator 326.
Actuator 326 is operable to tilt mirror element 322. In particular, support members 340 and torsion members 342 suspend mirror element 322 above substrate surface 328. Electrodes 344a and 344b are individually and separately charged (voltage source not shown) to attract mirror element 322. Torsion members 342 allow mirror element 322 to move through an angle, .+-..theta.. The position of mirror element 322 depends upon which of electrodes 344a or 344b is charged at a given moment. An optical signal (not shown) that is received by mirror element 322 is reflected to a different destination as a function of the tilt of that mirror element.
The aforedescribed mirror array substantially avoids the problematic coupled optical/mechanical response characteristic of the first arrangement. But, in avoiding that problem, other problems result. In particular, in the prior art mirror array, an actuator is required for each element of the array. The multiplicity of actuators in such an array significantly adds to its complexity and cost.
The art would thus benefit from adaptive optics in the form of a micro-machined mirror that avoids the optical/mechanical interdependence of the uniform reflective layer, and also avoids the multiple actuators of the conventional mirror array.